It finally occurs to him that he could try to yell for help. But he can't turn his head to look around - he can't see anybody else in the park, it's not that popular a park, there might not even be anybody - he can't open his mouth - he doesn't want to find out what'll happen if he tries and fails, if there's nobody around or they're not listening or they don't realize who's yelling or if he makes enough sound to let the bloodbender know he's trying but not enough for anyone else to hear it.

At least, that's what he hopes his reasons are. The same part of his mind that has inappropriate reactions to thinking about ripping people's bones out is having an inappropriate reaction to the sensation of being subtly, insidiously, helplessly puppeted by his own blood.